1. Field of the Invention
The invention relates to systems for controlling electronic warning devices which are more particularly intended as alarm devices.
2. Description of the Prior Art
Electronic alarm devices are usually formed by an astable multivibrator which operates at audio frequencies and which controls a power transistor, said power transistor driving one or several loudspeakers with square wave signals which are rich in harmonics.
When modulation of the audio frequency is required, the audio frequency is generally varied cyclically under the control of a modulator multivibrator.
Further, when it is necessary to limit the duration of the signal emission, a conventional timer system such as a capacitor which discharges through a resistor is generally used.
Identical (except for capacitor and resistor values) integrated circuits can be used for all three functions.
Generally, obtaining the audio frequency does not set any serious problems. In contrast, the modulation frequency and, a fortiori, the timer, are difficult to maintain at constant and repeatable values in industrial applications, since the values of capacitance required generally necessitate the use of electrolytic capacitors whose tolerances are very wide. Obviously, adjustable resistors can be provided, but adjustment thereof is long and increases the cost price of the equipment. It is difficult and often impossible to correct temperature variations.